It is known that in order to communicate with a mobile automatic station, for instance a meteorologic satellite, a communication satellite or a space probe, the ground-station antenna must constantly point in the direction of the object in motion, following all its movements. To facilitate this task, a radio-wave generator or radio beacon is placed aboard the automatic station; the beacon, by constantly radiating an electromagnetic signal toward the various stations spread over the earth's surface, allows them to position their antennas. This operation, generally referred to as "tracking", is automatically performed so as to ensure reliable operation at any time.
For this purpose, the ground stations are equipped with autotrack systems allowing to keep the direction of maximum reception of the antennas coincident with the direction of the beacon placed aboard the satellite.
This requirement is to be satisfied not only when the satellite is in motion with respect to the earth (non-geostationary satellite) but also when it is in an orbit in which it ought to be fixed with respect to the earth (geostationary satellite). In fact, the satellite is always under the gravitational forces exerted by the other celestial bodies of the solar system. The resultant of these forces causes an oscillation of the actual position of the satellite around its nominal position.
A radio-beacon tracking system must be able to detect these variations and therefore must send suitable corrective signals to the motors which position the antenna in both azimuth and elevation. These signals are obtained by duly extracting and processing the information relative both to the dominant propagation mode in the waveguide of the antenna feed and to the higher mode or modes corresponding to the configurations of the electromagnetic field that are generated owing to the pointing error.
The propagation modes to be utilized as well as the processing methods depend on the kind of polarization of the electromagnetic wave transmitted by the radio beacon. Thus, in satellites equipped with telecommunication apparatus operating in the range of frequencies from 4 to 6 GHz, communication radio beams as well as radio-beacon signals utilize circular polarization. A tracking system which extracts the error signals from the propagation mode TM.sub.01 in the guide has been already developed for such satellites.
For satellites equipped with telecommunication apparatus operating at frequencies higher than 10 GHz, the general trend is toward the use of signals radiated by linear polarization; for these signals the tracking system used so far for circularly polarized radio beacons is no longer suitable.
Present requirements are for tracking systems designed for linear polarization only, or for both linear and circular polarization. Of course, the latter systems are more in demand as they can be utilized indifferently with ground antennas operating in networks of links via satellite with circularly or linearly polarized radio beacons.
Conventional tracking systems suitable for both kinds of polarization can be basically grouped into two families: the first comprises systems in which the error signals are obtained from information associated with several higher modes in addition to the dominant one; the second comprises systems utilizing one higher mode only.
More particularly, the second family comprises systems utilizing the waveguide propagation mode TE.sub.21.
The solutions utilizing several higher modes are more complicated, because they require different kinds of mode couplers and because the processing necessary to extract error signals is more involved; furthermore, the overall adjustment of the equipment is more critical.
In the cases in which the tracking system utilizes only the mode TE.sub.21, a problem to be solved arises from the fact that the direction of the electromagnetic field radiated by radio beacons with linear polarization is different in the various zones of the earth. The known technical solutions can face these difficulties by means of expensive and sophisticated methods of processing radio-frequency signals.